The path of trace elements in a combustion process: from feed coal to ash products

Abstract:

Coal is an important component in enabling the worlds energy demands to be met. This is
largely due to the fact that coal is a relatively inexpensive fuel when compared to the other
options such as oil, gas and nuclear. In 2005 coal fired power generation accounted for 41% of
the worlds electricity supply with this figure expected to rise to around 46% by 2030 (Energy
Information Administration, 2009). Despite the world’s dependence on coal as a means of
producing electricity, the combustion of coal in coal fired plants has received international
scrutiny due to the pollutants (CO2, NOx, SOx and trace elements) generated from the
combustion of coal in a chain grate boiler. This research focuses on the trace elements released
by the combustion of coal and the partitioning behaviour of selected trace elements (As, B, Be,
Cd, Pb, Hg, Se, Cr, Ni, Sb, Co, Mn, Ur and Th). These trace elements were selected because they
are included in the US Clean Air Act Amendments Law of 1990 which states that they are
potentially toxic airborne pollutants and the lowering of their concentrations in the
environment is important (Aunela-Tapola et al. 1998). There is little or no available literature on
trace element partitioning and emissions in South Africa, despite the fact that there are
approximately 8000 industrial boilers in this country (personal communication, Falcon February
2010).
Inductively Coupled Plasma-Mass spectroscopy was used to obtain the concentration of the
selected trace elements in the four samples being investigated (feed coal, bottom ash, fly ash
and stack emission) which were obtained from a chain grate boiler. Proximate analysis and
particle size distribution (PSD) were performed to explain certain trends observed with the
trace element partitioning and emission results.
The results from the research show that the concentration of most of the trace elements
increases throughout the value chain i.e. from feed coal through bottom ash and flyash to stack
emission. Furthermore, the presence of highly volatile trace elements such as Hg and Se in the
bottom ash is best explained by the proximate analysis which indicates that unburnt coal is
present in the bottom ash. This indicates that the combustion of coal in the chain grate boiler in
this research is relatively inefficient. All trace elements were found to be highly enriched in the
stack emission which is due to the inefficient capture of particulate and vapour species by the
environmental control device/s (in this case, only an ESP). This was verified by the PSD results
which showed that particles of up to 17μm were present in the stack emission.
The research concludes that although industrial boilers combust lower amounts of coal than
larger pulverized fuel boilers, the trace element emissions from industrial boilers are significant.
This may be due to cost reduction practices resulting in the implementation of cheaper, less
efficient environmental control devices leading to the escape through the stack of a larger
concentration of trace elements adsorbed onto particulate matter.